the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
10Be age control of glaciation in the Beartooth Mountains, USA from the latest Pleistocene through the Holocene
- 1Department of Geology, Rowan University, Glassboro, NJ, USA
- 2Department of Geoscience, University of Wisconsin – Madison, Madison, WI, USA
- 3Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA, USA
- 4Department of Physics and Astronomy, Purdue University, West Lafayette, IN, USA
- 5Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
- 1Department of Geology, Rowan University, Glassboro, NJ, USA
- 2Department of Geoscience, University of Wisconsin – Madison, Madison, WI, USA
- 3Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA, USA
- 4Department of Physics and Astronomy, Purdue University, West Lafayette, IN, USA
- 5Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA
Abstract. Alpine glaciers in the western United States are often associated with late-Holocene Little Ice Age (LIA) advances. Yet, recent studies have shown many of these glacial landforms are remnants of latest-Pleistocene retreat with only the most cirque-proximal moraines preserving LIA activity. Additionally, the timing and magnitude of glacial advances during the Neoglacial-LIA interval remains uncertain with presumed maximum extents occurring during the LIA driven by lower Northern Hemisphere insolation levels. Here we present 10Be surface exposure ages from a glacial valley in the Beartooth Mountains of Montana and Wyoming, United States. These new data constrain the presence of the glacier within 2–3 km of the cirque headwalls by the end of the Pleistocene with implications for large-scale retreat after the Last Glacial Maximum. Cirque moraines from two glaciers within the valley preserve a late-Holocene readvance with one reaching its maximum prior to 2.1 ± 0.2 ka and the other 0.2 ± 0.1 ka. Age variability among the moraines demonstrates that not all glaciers were largest during the LIA and presents the possibility of regional climate dynamics controlling glacial mass balance.
Aaron M. Barth et al.
Status: open (until 28 Jul 2022)
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RC1: 'Comment on gchron-2022-17', Anonymous Referee #1, 04 Jul 2022
reply
This paper presents new cosmogenic 10Be exposure ages from moraines in the Glacier Lake Valley in the Beartooth Mountains of Montana/Wyoming. The ages span from the Younger Dryas (as interpreted) to very recent, and will provide a useful new chronology for moraines in this location.
However, I do have concerns about the paper in its present form. The manuscript feels like a draft and it requires some significant work before it can be acceptable for publication. First, there are inconsistencies in the way ages are calculated, reported, and interpreted throughout the paper. I have elaborated on these issues below, but they make the paper confusing and difficult to follow at present. Second, I am concerned that the precision of the ages is over-interpreted, and specifically that the attempt to place the Emerald Lake moraine within the Younger Dryas with centennial-scale precision is not warranted. Third, the presentation of the paper needs improvement, including both the writing and the figures. I am confident that all of these issues can be addressed and the manuscript can become acceptable for publication, but the changes required are Major because they touch on all parts of the paper and the underlying age calculations.
My primary concern is that the exposure ages underpinning the study are unclear. The paper seems to be mixing production rates (that are not explained in the Methods), and discussing ages in the text that contradict those in the figures. In addition, there is some confusion about how clusters of boulder ages are being interpreted, and I am concerned that different approaches are taken for different clusters.
The text in the Results describes ages that do not match Fig. 1 (and are not reported in Table 1). For example, at L218 the ages for the Emerald Lake moraine are reported as between 11.3 and 13.0 ka, but on Fig 1 are reported as between 10.8 and 12.1 ka. This is a recurring source of confusion throughout the paper, affecting all the ages and interpretations. The text around L223-224 indicates that different production rates are being used – why is this not explained in the Methods? Mixing production rates is confusing and inappropriate. My understanding is that Fig. 1 is using an old production rate from 1995, as indicated at L223 (because the mean age of 11.6 matches Fig. 1). If this is correct, then a better approach would be to select the most appropriate production rate, use it consistently so the text and figures match, and somewhere within the paper address how the uncertainty on the production rate propagates into uncertainty on the exposure ages.
I’m sure this confusion can be resolved by calculating the ages consistently throughout the paper. However, this does lead me to some further concerns about how the clusters of ages are interpreted.
I am concerned that section 5.2 overstates what can be interpreted from the ages given their uncertainties. It’s also not clear what the purpose of this section is. If it’s to suggest that the Emerald Lake moraine likely coincides with the Younger Dryas, then I agree but this point probably does not require a section. If it’s to interpret where the ages fall within the Younger Dryas, then I do not think this is possible given the uncertainties on the dating. Again, the ages need to be sorted out. At L326-328 two scenarios are offered: deposition at the start or throughout the YD, but the mean age given in Fig. 1 is 11.6 ka, i.e., just after the YD entirely, which seems contradictory. The next paragraph opens giving the average age as 12.5 ka, and this magnitude of discrepancy undermines any attempt to place the moraine inside the YD interval at a sub-millennial resolution. The age given in Fig. 1 then appears at L342 and the data are reinterpreted again, which feels like reading two different papers in parallel. Either way, I don’t think the age precision warrants an interpretation of when the moraine formed within the YD. This claim comes up again in the Conclusion at L413 (“early in the Younger Dryas”).
Along the same lines, I am sceptical about the claim that exposure ages show regional variability in timings within the Younger Dryas (paragraph ending at L341). If the uncertainties on the ages calculated using different production rates are as large as the duration of the YD itself, I am not convinced that we can resolve much finer-resolution differences in timing across the region.
Aside from the production rates, I am concerned that something does not add up in the way the clusters of boulder exposure ages are being interpreted. Figure 6 is the issue here – the ages for LGLI given in the text are a little older than those shown in Fig. 1, yet the point for LGLI in Fig. 6 is placed younger than the mean age given for LGLI. I understand that different production rates are being mixed, but I think there is an additional difference in the way LGLI and LGLO are being handled. Please clarify. Is Fig. 6 plotting the mean age for LGLO but the youngest age for LGLI? It’s not clear what is happening here because L229-231 states that none of the LGLI ages are rejected as outliers, but on Fig. 6 a very precise and comparatively young age is used instead. Perhaps I am missing something, but if so then the text needs to be clearer.
Subsequently, the text claims that the young moraine ages correlate with the lacustrine records presented in Fig. 6 (L386-387), but it doesn’t look like it to me. The LGLI age plotted seems to fall in between the two main shaded peaks in the blue CaCO3 flux curve. Which itself seems to be somewhat anti-correlated with the orange %VF silt curve?
Those are my major concerns, and my recommendation is that they need to be addressed before the paper can be acceptable. Below are more minor comments that I hope will help to improve the paper's clarity and impact.
Some of the referencing could give credit to earlier, classic studies. For example around L212; work was done before Barth et al. (2019) to suggest that younger exposure ages from moraines are likely to be biased by incomplete-exposure effects (e.g., erosion, exhumation, toppling, etc). Some original citations could be acknowledged here. Also at L268-270; important work was done before that of Lora and Ibarra, attributing past changes in North American hydroclimate to migration of the jet stream. To my understanding this hypothesis was presented as early as the 1980s. Please also ensure that the referencing is done properly, e.g., I can’t find Osman et al. (2021) in the reference list, but it is cited in the figure captions.
The Methods could be written more precisely:
- L183: How concentrated/dilute were the acids?
- L185: How many rounds of HF/HNO3 etches were performed?
- L200: Specify the reference production rate used – what is the actual rate in atoms/g/yr? Furthermore, I advise against using multiple production rates throughout the paper, but if you’re going to do that then please explain it in the Methods.
- L204: Do Ballantyne and Stone (2012) constrain boulder-surface erosion rates between 0 and 0.1 cm/kyr? I am not sure they do.
Much of the writing could be improved, and the paper needs a careful check for readability, language and grammar. Some illustrative examples:
- “Glacial” is used in quite a few places where “glacier” would be more appropriate, e.g. L22, L25, L26, L284.
- L25: “clearest indicators of the climate system’s response to recent global warming”. This is an odd sentence because warming is part of the climate – rephrase.
- L26: “Photographic and satellite imagery of reductions…”. This could be phrased better, e.g., “photographs and satellite imagery of glacier extents from the past century document widespread retreat”.
- L35: “the Holocene *was* considerably more stable”
- L36: “suggests other mechanisms *controlled glacier size* besides NH insolation”
- L57: “Beartooths” – this is colloquial, change to Beartooth Mountains.
- L62: Change “course” to “coarse-crystalline”
- L298-302: this sentence does not work grammatically.
- L318: Write out “Younger Dryas” before using the abbreviation.
- L376 – grammar needs checking.
- L398: highlight*ed* - past tense
- L403-404: “Similar influence of topography have been shown to influence” – rewrite this.
The figures require some improvements:
- 1 is missing lat/long coordinates and a legend for the colour scale. It would be more helpful for the inset map to show a box or star locating the study area specifically, rather than just colouring in the two states. I recommend making the solid-black moraine lines a bit bolder so that they stand out.
- 2 also needs lat/long coordinates. I suggest outlining the white boulder dots, and also enlarging this figure so it takes up the full page width.
Line comments:
L28-29. I disagree that analysis of glacier sensitivity to climate change is limited by the instrumental record, which only goes back decades. Many studies have dated ancient moraines - as this one does – to constrain glacier sensitivity to climate changes of much larger magnitudes than those provided by instrumental records.
L218. The text refers the reader to Table 1 for the ages, but the ages are missing in Table 1. Figure 1 doesn’t help either because the ages reported there aren’t given sample names, so they can’t be cross-referenced. Same issue with L 229-230 and elsewhere. Add the ages to Table 1.
L231-239. This is all interpretation and discussion, which shouldn’t really be mixed with the Results. Move to the Discussion. I would also question whether a population of only four ages can be interpreted as having a multi-modal distribution.
L366. I think this should refer to Fig. 5, as insolation is not shown in Fig. 6.
L383. Figure 6, not 5?
L406-407. This would place deglaciation within historical records between about 1647 and 1899. Are there any historical accounts of this?
Aaron M. Barth et al.
Aaron M. Barth et al.
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